This invention relates to a method of manufacturing a liquid crystal display device of the kind having a matrix of individually operable liquid crystal picture elements, the method comprising the steps of forming on a transparent substrate at least one set of opaque, substantially parallel address conductors, a matrix array of individual picture element electrodes, and switching elements each of which is located adjacent a respective picture element location, and electrically connecting the picture element electrodes to the switching elements, and to the address conductors.
The invention relates also to liquid crystal display devices manufactured in accordance with such a method.
Active matrix addressed liquid crystal display devices are suitable for displaying alpha-numeric or video, for example TV information. The display devices may typically consist of a very large number of picture elements, possibly 200,000 or more.
In a known example of a liquid crystal display device suitable for displaying TV pictures and using thin film transistors, TFTs, as the switching elements, the picture elements are arranged in a matrix of rows and columns and are defined by liquid crystal material disposed between opposing substrates and by respective driving electrodes on one substrate and opposing portions of a common electrode carried on the other substrate. The TFTs are located laterally adjacent the driving electrode of their respective picture elements on the one substrate, with the drain electrode of each TFT connected to the associated driving electrode. The source electrode of all TFTs in the same column are connected to a respective one of a set of column address conductors extending between adjacent columns of picture elements to which data signals are applied. The gate electrodes of all TFTs in the same row are connected to a respective one of a set of row address conductors extending between adjacent rows of picture elements to which switching (gating) signals are applied. The device is driven by repetitively scanning the row conductors one at a time in sequential fashion so as to turn, on all TFTs in each row in turn and by applying data signals to the column conductors appropriately in synchronism for each row of picture elements in turn so as to build up a display. When the TFTs are in their on state the data signals are supplied to the associated picture element driving electrodes, thus charging up the picture elements. When the TFTs are turned off, upon cessation of the row scan signal, charge is stored in the picture elements concerned until the next time they are addressed with a row scan signal, which usually in the case of a video display is in the next field period.
Another type of known active matrix liquid crystal display device uses two-terminal non-linear elements, for example diode structures such as back-to-back diodes, diode rings or MIM (Metal-Insulator-Metal) devices, as the switching elements. As before, the picture elements are arranged in a matrix array of rows and columns. However, in these devices one set of address conductors, the row scanning conductors, is carried on one substrate and the other set of address conductors, the data column conductors, is carried on the other substrate. The picture elements are defined by individual picture element electrodes carried on one of the substrates between which address conductors extend and overlying portions of the address conductors carried on the other substrate. The individual picture element electrodes are each connected to their associated address conductor via a non-linear switching element which is arranged laterally of the electrode. The non-linear switching elements exhibit a threshold characteristic and the application of scan and data voltages to the set of address columns exceeding this threshold causes charging of the picture elements. As before, the picture elements are addressed sequentially a row at a time so as to build up a display.
Liquid crystal display devices of both types are operated in the transmissive mode whereby the individual picture elements act as shutters to control the transmission of light from a light source situated on the side of the device opposite the side where the generated display is viewed. The display devices have areas that are opaque, for example the areas occupied by the switching elements and the sets of address conductors (assuming these are formed of opaque conductive material), areas in which the intensity of transmitted light is controlled, that is active areas, determined by the areas of the picture element electrodes, and transparent areas which are uncontrolled.
The proportion of the display device's overall area that is active should be maximised to give optimum display brightness. This is particularly important in display devices intended for use in projection systems where light is directed onto one side of the device, modulated by the device in accordance with the picture to be displayed and then projected onto a display screen via a projection lens, as the physical size of the individual picture elements in such devices is comparatively small.
In use of the display devices, it has been found also that the uncontrolled transparent areas result in a general loss of contrast.